The present invention relates to an extraction method for separating petroleum-containing materials into at least two fractions, and also relates to an extraction system, the solvent stripping system and an extraction fluid therefor.
An economical method to de-asphalt, fractionate and de-metalize various petroleum-containing materials is needed. The petroleum-containing materials can, for example, be waste/non-waste petroleum-containing materials such as used motor oil, virgin crude oil, vacuum tower bottoms, catalytic cracker tower bottoms, heavy oil, gas oil, tar sands/bitumin and the like.
Liquid carbon dioxide is cheap, plentiful and relatively benign in terms of toxicity and its effects on the environment when compared to other solvents. Unfortunately, carbon dioxide is a very poor solvent in the sub-critical or liquid phase, and it has little utility as an extraction fluid, especially for petroleum mixtures. Auerbach (U.S. Pat. No. 1,805,751) states that sub-critical carbon dioxide dissolves petroleum oils to form solutions of concentrations of 2 percent or less. Such extractions are highly inefficient because of the need to use and process large amounts of solvent to effect an appreciable quantity of oil.
Because sub-critical carbon dioxide is a poor solvent, recent research utilizing carbon dioxide as a solvent has been concentrated on the super-critical phase. The term super-critical as used in conjunction with a fluid refers to a highly compressed gas having a gas density approaching that of the liquid phase density. A super-critical fluid cannot be liquefied. A super-critical fluid can also be defined as a substance which is above the critical temperature and at a pressure above the critical pressure. The critical temperature is defined as the temperature above which a gas can never become a liquid regardless of the pressure. The critical pressure is defined as the pressure at which a gas can just be liquefied at the critical temperature. There are few or no intermolecular attractions (liquid bonds) in a super-critical fluid and therefore it will expand to fill the entire container. A super-critical fluid has no meniscus and the density of the fluid is constant throughout the container. This is contrasted with a true liquid which has a meniscus with the liquid phase (high density phase) at the bottom of the container and the vapor phase (low density phase) at the top of the container.
In modern chemical theory, a super-critical fluid is considered as a separate phase along with solids, liquids and gases. A phase diagram consisting of a temperature vs pressure plot shows lines dividing the solid, liquid and gaseous phases and a xe2x80x9ctriple pointxe2x80x9d where the three phases are in equilibrium. The same plot may also show the critical point where the liquid, gas and the super-critical phases are in equilibrium. Usually the boundary lines between the gas, liquid and super-critical phases are dashed because of the difficulty of measuring those boundaries. Even with this difficulty, it is well recognized that a super-critical fluid is a distinctly different phase from that of solids, liquids and gases.
Since there are few or no inter-molecular bonds or attractions in a super-critical solutions, the rule of liquid solubility (polar solvents dissolve polar solutes well and nonpolar solvents dissolve nonpolar solutes well) tends to be minimized. The general rule of solubility for super-critical solutions is solvents and solutes of like densities are soluble, which is very different from that of liquid solvent systems. To illustrate the dramatic differences in solubility characteristics between super-critical solvents and sub-critical solvents these examples are cited. Super critical water and oil are miscible. Elemental carbon is soluble in super-critical toluene. These examples indicate that it is improper to make inferences about a solution in one phase by utilizing empirical data from another phase. It is very important to consider which material is the solvent and the solute when considering systems where the solvent is of a different phase from that of the solute.
Despite claims to the contrary, carbon dioxide in the super-critical phase is actually not an exceptional solvent either. For this reason, Ohgaki et al, U.S. Pat. No. 5,138,075, starts at a somewhat sub-critical phase before heating to the super-critical phase. In addition, polar co-solvents such as water, methanol, and ethanol ( less than 10%) have been added to super-critical carbon dioxide to increase the solubilities of potential solutes (see Dedieu et al, U.S. Pat. No. 5,329,045). Low volatile non-polar co-solvents have been added to carbon dioxide for extractions (see Heidlas et al, U.S. Pat. No. 5,626,756). Non-volatile surfactants have been used with sub and super-critical carbon dioxide in attempts to develop a commercial dry-cleaning process for clothing. The solubilities of non-gaseous materials in super-critical carbon dioxide are not great enough to be commercially useful except for very specialized food, medical and scientific applications. Caffeine extractions of coffee and tea, drug and drug precursors are extracted commercially utilizing super-critical carbon dioxide.
Kriegel, U.S. Pat. No. 4,522,707, discloses the use of gases, including carbon dioxide, at super-critical conditions for processing spent oil. There are several other patents pertaining to the use of super-critical carbon dioxide in the oil industry. Harris et al., U.S. Pat. No. 5,045,220 notes that carbon dioxide easily associates with various polymers, various light hydrocarbons and water to facilitate tertiary recovery of oil from oil fields.
Older patent references often referred to super-critical fluids as liquids. Francis (U.S. Pat. No. 2,631,966), for example, presented extensive solubility data and extraction methods for virgin lubricating oils utilizing carbon dioxide and various co-solvents including propane. The various solvent systems are referred to as liquids although the data and descriptions are often presented for temperatures and pressures greater than the super-critical conditions of the solvent.
Francis describes the conditions of most of his extractions with regard to a Plait point which describes what he refers to as the critical solution point. His critical point represents the solution conditions where two liquid phases in an extraction experiment disappear when the compositions of the phases approach each other through the variation of the solvation parameters. Great care is required in the interpretation of terminology used in the old literature.
Francis describes extractions involving a (type A) co-solvent as a co-solvent which is completely miscible in liquid carbon dioxide and partially miscible in the mixture to be separated. The second (type B) extraction involves a co-solvent which is partially soluble in carbon dioxide and partially miscible in the mixture to be separated.
The patent omits another type of extraction which would have a co-solvent which is completely miscible in carbon dioxide and also completely miscible in the oil. This case is the substance of our inventive process. The Francis extractions result in fractions which he terms extract-extract, extract-raffinate, raffinate-extract and raffinate-raffinate. The co-solvent and the carbon dioxide are separated after the extractions and solvents are reformulated before reuse. Our inventive process simply flash vaporizes and recycles the solvent and co-solvent without the separation of the solvent components and without the need for reformulation of the solvent upon reuse.
Liquid propane in the sub-critical phase is used to de-asphalt petroleum materials via salvation techniques (see Mellen, U.S. Pat. No. 5,286,380, Crowley, U.S. Pat. No. 4,169,044, Wezel, U.S. Pat. No. 4,797,198, and Vu, U.S. Pat. No. 3,773,658). Propane easily solvates oils which are removed from a mixture then heated to the super-critical phase to lower the solubility and to recover the oils. Small quantities of carbon dioxide, hydrogen sulfide, etc. have been added to modify or lower the solvent properties of propane. (see, for example, Yoon et al, U.S. Pat. No. 5,587,085, and Heidlas et al, U.S. Pat. No. 5,616,352). It should be noted that propane as an extraction fluid is too good of a solvent to efficiently fractionate petroleum mixtures. Lipid oils and cholesterol from various sources have been extracted using pure propane and propane with small quantities of co-solvents, such as carbon dioxide.
Van Dijck (U.S. Pat. No. 2,281,865) utilized extraction separations by commingling a pure solvent with a petroleum mixture. Various low molecular weight, high volatile solvents including pure propane and pure carbon dioxide were utilized. After an equilibrium solution was reached the pressure was released step-wise resulting in the settling of a high molecular weight fraction layer. The separated layer was removed before the pressure was lowered to the next step. The step-wise lowering of the pressure and the resultant requirement for the formation of a new equilibrium restricted the separation to a batch process method. The temperatures of these extractions were usually below but near the critical temperatures of the solvent.
Webb (U.S. Pat. No. 2,246,227) diluted lubricating oils with propane and then treating the resulting solutions with methane to produce separations of oils into fractions of different densities.
Lantz (U.S. Pat. No. 2,188,051) utilized low molecular weight hydrocarbon solvents (7 carbons or less) to solvate the oil under consideration. The solvent properties of the light hydrocarbon are then modified with the addition of carbon dioxide to form petroleum fractions of lower and higher viscosities. He further states that the extractions work best utilizing branched chain hydrocarbon solvents such as isobutane and isopentane solvents and in solvent concentrations of 75% or more. The recovery of the solvent components requires separate systems to recover the carbon dioxide and the hydrocarbon solvent.
It is an object of the present invention to provide an improved and economical method to de-metalize, de-asphalt and fractionate petroleum-containing materials such as used motor oil or virgin light crude oil, heavy crude oils and tar sands/bitumen using specific sub-critical solvent/co-solvent mixtures.
It is another object of the present invention to clean diesel fuels, fuel oils, aviation gasoline and other fuels.
It is a further object of the present invention to clean and separate oil from earthen materials after oil spills as well as to clean oil from plastics prior to plastic recycling.
It is also an object of the invention to separate the constituents of extremely viscous materials such as cracker tower bottoms and gas oils.
It is a further object of the present invention to provide an extraction system to carry out such a method.
It is another object of this invention to efficiently separate the solvent/co-solvent mixture from the extract products in a simple single step process while maintaining a constant solvent to co-solvent ratio.
It is also an object of this invention to utilize much of the heat of vaporization obtained from the solvent recovery system to vaporize the solvent in the solvent stripping system.